This invention relates to a secondary battery protection circuit for use in a battery unit comprising at least one chargeable electric cell (secondary battery) such as a lithium ion cell and, in particular, to a secondary battery protection circuit comprising an overdischarge prevention circuit and an overcharge prevention circuit.
Among various types of chargeable electric cells, a lithium ion cell is particularly weak against overdischarge or overcharge. In this connection, it is essential to provide a secondary battery protection circuit for detecting an overdischarge condition and an overcharge condition to protect a secondary battery from the overdischarge condition and the overcharge condition. For this purpose, the secondary battery protection circuit comprises an overdischarge prevention circuit and an overcharge prevention circuit. In addition, the secondary battery protection circuit may detect an overcurrent condition during discharge of the secondary battery to protect the secondary cell from the overcurrent condition. In this event, the secondary battery protection circuit may comprise an overcurrent prevention circuit as well as the overdischarge prevention circuit and the overcharge prevention circuit. However, description will be hereinuder directed to the secondary battery protection circuit comprising the overdischarge prevention circuit and the overcharge circuit.
Such a secondary battery protection circuit is disclosed, for example, in Japanese Granted Patent Publication of No. 2,872,365 or JP-B 2872365 (which will later be called xe2x80x9cprior art referencexe2x80x9d) under the title of xe2x80x9cCHARGEABLE POWER UNIT.xe2x80x9d According to JP-B 2872365, the secondary battery protection circuit comprises a secondary battery, a discharge control switch, and a charge control switch. The discharge control switch and the charge control switch comprise first and second MOSFETs containing first and second parasitic diodes, respectively. The first parasitic diode is connected to the first MOSFET so that a forward direction of the first parasitic diode coincides with a charging direction of the secondary battery. The second parasitic diode is connected to the second MOSFET so that a forward direction of the second parasitic diode coincides with a discharging direction of the secondary battery.
However, in the manner which will later be described in conjunction with FIGS. 1 and 2 in detail, it is feared that the following problems occur in the above-mentioned prior art reference. That is, energy is consumed in the charge control switch (the second MOSFET) for a time duration from a time instant when a load is connected between external connection terminals and to a time instant when the second MOSFET is turned on. This is because the discharging current flows through the second parasitic diode in the second MOSFET for this time duration. As a result, if this time duration is too long, heat generates in the second MOSFET and it is therefore feared that destruction of the second MOSFET is caused by the generated heat. Likewise, energy is consumed in the discharge control switch (the first MOSFET) for a time duration from a time instant when a charger is connected between the external connection terminals to a time instant when the first MOSFET is turned on. This is because the charging current flows through the first parasitic diode in the first MOSFET for this time duration. As a result, if this time duration is too long, heat generates in the first MOSFET and it is therefore feared that destruction of the first MOSFET is caused by the generated heat.
It is therefore an object of the present invention to provide a secondary battery protection circuit which is capable of prevent a charge control switch from destroying.
It is another object of the present invention to provide a secondary battery protection circuit which is capable of prevent a discharge control switch and a charge control switch from destroying.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of an aspect of this invention, it is possible to be understood that a method is for protecting a secondary battery by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger connected between external connection terminals. The secondary battery generates a battery voltage.
According to the aspect of this invention, the above-mentioned method comprises the steps of turning the charge control switch off when the battery voltage is higher than an overcharge detection threshold voltage, and of immediately turning the charge control switch on when a load for the charger is connected between the external connection terminals.
On describing the gist of another aspect of this invention, it is possible to be understood that a secondary battery protection circuit is for protecting a secondary battery by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger connected between external connection terminals. The secondary battery is connected between a power source terminal and a ground terminal to generate a battery voltage.
According to the other aspect of this invention, the above-understood secondary battery protection circuit comprises a switch off arrangement for turning the charge control switch off when the battery voltage is higher than an overcharge detection threshold voltage. A switch on arrangement is for immediately turning the charge control switch on when a load for the charger is connected between the external connection terminals.
On describing the gist of still another aspect of this invention, it is possible to be understood that a method is for protecting a secondary battery by controlling, by turning a discharge control switch on and off, a discharging current flowing from the secondary battery through a load connected between external connection terminals and by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger connected between the external connection terminals. The secondary battery generates a battery voltage.
According to the still another aspect of this invention, the above-understood method comprising the steps of, on discharging where the load is connected between the external connection terminals or on naturally discharging, turning the discharge control switch off when the battery voltage is lower than a predetermined overdischarge detection threshold voltage, of immediately turning the discharge control switch on when the charger for the load is connected between the external connection terminals, of, on charging where said charger is connected between the external connection terminals, turning the charge control switch off when the battery voltage is higher than a predetermined overcharge detection threshold voltage, and of immediately turning the charge control switch on when the load for the charger is connected between the external connection terminals.
On describing the gist of yet another aspect of this invention, it is possible to be understood that a secondary battery protection circuit is for protecting a secondary battery by controlling, by turning a discharge control switch on and off, a discharging current flowing from the secondary battery through a load connected between external connection terminals and by controlling, by turning a charge control switch on and off, a charging current flowing through the secondary battery from a charger connected between the external connection terminals. The secondary battery is connected between a power source terminal and a ground terminal to generate a battery voltage.
According to the yet another aspect of this invention, the above-understood secondary battery protection circuit comprises a discharge switch off arrangement, on discharging where the load is connected between the external connection terminals or on naturally discharging, for turning the discharge control switch off when the battery voltage is lower than a predetermined overdischarge detection threshold voltage. A discharge switch on arrangement is for immediately turning the discharge control switch on when the charger for the load is connected between the external connection terminals. On charging where the charger is connected between the external connection terminals, a charge switch off arrangement is for turning the charge control switch off when the battery voltage is higher than a predetermined overcharge detection threshold voltage. A charge switch on arrangement is for immediately turning the charge control switch on when the load for the charger is connected between the external connection terminals.